Our long term goal is to investigate the biochemical mechanism involved in cataractogenesis using 3-amino-1H-1,2,4-triazole (AT) induced cataract in rabbit as an experimental model. This compound is a specific inhibitor of catalase. We demonstrated that catalase of eye plays an important role in regulating the endogenous peroxide of aqueous humor and vitreous humor to physiological levels, and we speculated that H2O2 may be the triggering factor in cataract-induced by AT. Our recent studies on cation transport in early cataract (vacuolar stage) induced by paired feeding (2 wks) of AT in littermate young Dutch rabbit show no change in 86Rb ion uptake, 50% increase in 86Rb ion efflux and a significant increase in extracellular space (D-Mannitol). Influx of 40 mM 22Na ion was increased 4X and efflux was decreased by 16% (P equals less than .01). This indicates that in early cataract the passive fluxes of cations are markedly increased. Among the other changes observed in early cataract were 65% decrease in ascorbic acid of aqueous humor and vitreous humor, 40% decrease in ascorbic acid of lens and no change in GSH of lens. Both ascorbic acid and GSH constitute an antioxygenic potential in lens. The results described show a change in ascorbic acid of lens at a stage when GSH of lens was unaltered. We observed 2X increase in H2O2 concentration in aqueous humor and vitreous humor in early cataract. Significant activities of superoxide dismutase (SOD) were observed in lens, iris, ciliary body, retina and cornea. We have also evidence that endogenous catalase of lens affords protection to SOD from inhibitory effect of H2O2. Further studies to examine the sources of H2O2 in eye and to investigate the detail mechanism by which H2O2 causes oxidative damage to the lens are in progress. We also propose to evaluate the activities of SOD, catalase and GSH peroxidase in cataractous lenses of the human.